Several applications may require that the outermost surface of an integrated circuit die be exposed to the environment, such as to be accessible for direct contact with a person or part of a person. For example, certain medical applications position a bare integrated circuit into the blood stream, such as to measure blood chemistry. Fortunately, in such an application the circuit's operational life is limited, and the circuit is used only once before being discarded. Accordingly, long term reliability is not typically a significant issue.
In contrast, a fingerprint sensor based upon an integrated circuit array of sensing elements may require direct contact by the finger with the integrated circuit die. Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. One common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in storage to determine proper identification of a person, such as for verification purposes.
A typical electronic fingerprint sensor is based upon illuminating the finger surface using visible light, infrared light, or ultrasonic radiation. The reflected energy is captured with some form of camera, for example, and the resulting image is framed, digitized and stored as a static digital image. For example, U.S. Pat. No. 4,210,899 to Swonger et al. discloses an optical scanning fingerprint reader cooperating with a central processing station for a secure access application, such as admitting a person to a location or providing access to a computer terminal. U.S. Pat. No. 4,525,859 to Bowles similarly discloses a video camera for capturing a fingerprint image and uses the minutiae of the fingerprints, that is, the branches and endings of the fingerprint ridges, to determine a match with a database of reference fingerprints.
Unfortunately, optical sensing may be affected by stained fingers or an optical sensor may be deceived by presentation of a photograph or printed image of a fingerprint rather than a true live fingerprint. In addition, optical schemes may require relatively large spacings between the finger contact surface and associated imaging components. Moreover, such sensors typically require precise alignment and complex scanning of optical beams. Accordingly, optical sensors may thus be bulky and be susceptible to shock, vibration and surface contamination. Accordingly, an optical fingerprint sensor may be unreliable in service in addition to being bulky and relatively expensive due to optics and moving parts.
U.S. Pat. No. 4,353,056 to Tsikos discloses another approach to sensing a live fingerprint. In particular, the patent discloses an array of extremely small capacitors located in a plane parallel to the sensing surface of the device. When a finger touches the sensing surface and deforms the surface, a voltage distribution in a series connection of the capacitors may change. The voltages on each of the capacitors is determined by multiplexor techniques. Unfortunately, the resilient materials required for the sensor may suffer from long term reliability problems. In addition, multiplexing techniques for driving and scanning each of the individual capacitors may be relatively slow and cumbersome. Moreover, noise and stray capacitances may adversely affect the plurality of relatively small and closely spaced capacitors.
U.S. Pat. No. 5,325,442 to Knapp discloses a fingerprint sensor including a plurality of sensing electrodes. Active addressing of the sensing electrodes is made possible by the provision of a switching device associated with each sensing electrode. A capacitor is effectively formed by each sensing electrode in combination with the respective overlying portion of the finger surface which, in turn, is at ground potential. The sensor may be fabricated using semiconductor wafer and integrated circuit technology. The dielectric material upon which the finger is placed may be provided by silicon nitride or a polyimide which may be provided as a continuous layer over an array of sensing electrodes. Further conductors may be provided on the surface of the dielectric material remote from the sensing electrodes and extending over regions between the sensing electrodes, for example, as lines or in grid form, which conductors are grounded in order to improve the electrical contact to the finger surface.
Unfortunately, driving the array of closely spaced sensing electrodes as disclosed in the Knapp et al. patent may be difficult since adjacent electrodes may affect one another. Another difficulty with such a sensor may be its ability to distinguish ridges and valleys of a fingerprint when the conductivity of the skin and any contaminants may vary widely from person-to-person and even over a single fingerprint. Yet another difficulty with such a sensor, as with many optical sensors, is that different portions of the fingerprint may require relatively complicated post image collection processing to provide for usable signal levels and contrast to thereby permit accurate determination of the ridges and valleys of the fingerprint.
Greater advances in fingerprint sensing and matching for identification and verification are desirable and may prevent unauthorized use of computer workstations, appliances, vehicles, and confidential data. Inexpensive and effective fingerprint identification may also be used at point-of-sale terminals, and ensure further security of credit and debit cards, firearms, and provide a personal electronic signature. Unfortunately, current sensors and their associated circuitry may be too bulky, expensive and unreliable for a great many applications which would otherwise benefit from fingerprint identification and verification technology.
Even though several integrated circuit approaches to fingerprint sensing have been described, there are significant difficulties in packaging such a sensor. As mentioned above, direct physical contact with an integrated circuit die is counter to conventional integrated circuit manufacturing and packaging concepts. Water vapor and other contaminants may enter any interface between the integrated circuit die and the surrounding package thereby potentially significantly reducing reliability. Moreover, even if a reliable package were to be designed, low cost mass production techniques would still be desirable for implementing the packaging design. Other sensors and integrated circuit devices may also desirably benefit from packaging advances which provide an integrated circuit device including a package which exposes the integrated circuit die, yet which provides environmental protection for other portions of the die, such as the bond pads and bond wires.